How air changed: proportions, evidence and carbon stores
Introduced| English | Meaning |
|---|---|
| volcanic activity | Volcanic eruptions and related processes; release of volcanic gases is one proposed explanation for Earth’s early atmosphere. |
| condensation/kɒndenˈseɪʃn/ | The change of a substance from a gas to a liquid. |
| algae/ˈælɡiː/ | A diverse group of mainly aquatic photosynthetic organisms that lack the true roots, stems and leaves of plants. |
| photosynthesis/ˌfəʊtəʊˈsɪnθəsɪs/ | The process by which plants and other organisms use light energy to make organic substances from carbon dioxide and water. |
| fossil fuel/ˈfɒsl ˈfjuːəl/ | Coal, oil and natural gas formed from ancient organic matter over millions of years. |
| limestone/ˈlaɪmstəʊn/ | A rock of mainly calcium carbonate, used for building and in the blast furnace. |
| coal/kəʊl/ | A fossil fuel formed from compressed ancient plant material. |
| crude oil/kruːd ɔɪl/ | A naturally occurring liquid mixture containing many hydrocarbons and smaller amounts of other substances. |
| natural gas/ˈnætʃərəl ɡæs/ | A fossil fuel mixture, mainly methane. |
Why is today's air unlike volcanic gas?
- Today's air is roughly four-fifths nitrogen and one-fifth oxygen, with small amounts of other gases. Water vapour varies; dry-air proportions exclude it.
- Volcanoes release much carbon dioxide, but it is a small component of today's air. Explain the processes that changed the atmosphere rather than treating today's air as a direct volcanic sample.
Convert a gas fraction into a volume
- For a fixed temperature and pressure, gas volume = its fraction × total air volume. Use one consistent dataset: the rough 80/20 model deliberately ignores small components.
- Worked case: 300 cm³ of air in that model contains $\frac45\times300=240$ cm³ nitrogen and $\frac15\times300=60$ cm³ oxygen. A separate supplied 78/21/1 dataset must be calculated with its own fractions.
For a 500 cm³ sample with 78% nitrogen, 21% oxygen and 1% other gases, the nitrogen volume is $0.78\times500=390$ cm³. The other gases total 5 cm³; they are not all carbon dioxide.
A 500 cm³ sample is represented by 78% nitrogen. Calculate its nitrogen volume at the same temperature and pressure.
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Nitrogen volume = nitrogen fraction × total volume = 78/100 × 500 cm³ = 390 cm³. Use 0.78, not 78 or the different rough 4/5 model.
Separate the early-atmosphere theory from certainty
- One theory links the early atmosphere to intense volcanic activity 火山活动 during Earth's first billion years: mainly carbon dioxide, water vapour and nitrogen, with little or no oxygen and small amounts of methane/ammonia.
- Earth formed about 4.6 billion years ago. Direct measurements from then do not exist, and evidence is limited. Modern volcanic gases, rocks and other planets can support a theory; they do not supply an exact ancient percentage. Ice cores record later air, not the first atmosphere.
Which conclusion is supported by observing CO₂ from a modern volcano?
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The observation supports a possible gas source. It neither measures an ancient percentage nor establishes today’s oxygen proportion for the earliest atmosphere.
Ancient ice directly samples the atmosphere when Earth first formed.
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Ice records later atmospheres, not air from Earth’s formation about 4.6 billion years ago. The first atmosphere must be inferred from limited indirect evidence.
Follow water and carbon by different routes
- Condensation 凝结 changes water vapour into liquid as Earth cools, forming oceans. It does not change water into carbon dioxide.
- Carbon dioxide dissolves in ocean water; carbonates precipitate and form sediments. These processes remove carbon from the atmosphere. Nitrogen from volcanic activity gradually builds up.
Explain the biological oxygen increase
- Algae 藻类 first produced oxygen by photosynthesis 光合作用 about 2.7 billion years ago; plants evolved later. Oxygen rose gradually over the next billion years to a level that enabled animals to evolve.
- Photosynthesis uses light: $6CO_2+6H_2O\rightarrow C_6H_{12}O_6+6O_2$. Carbon dioxide enters biomass and oxygen is released. Each side has six carbon, twelve hydrogen and eighteen oxygen atoms; respiration has the opposite overall gas effect.
Which process used by algae removed CO₂ and released oxygen?
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Photosynthesis uses carbon dioxide and water to form glucose and oxygen using light. Algae began contributing oxygen before plants evolved.
Distinguish rock from fossil-fuel 化石燃料 formation
- Limestone 石灰岩 forms from accumulated carbonate sediments, including marine shells/skeletons. Coal 煤 forms from buried plant remains; crude oil 原油 and natural gas 天然气 form mainly from buried small marine organisms changed by heat and pressure over millions of years.
- Burial can store carbon outside the atmosphere for a long time rather than returning all of it by decay. These fossil fuels do not form instantly when carbon dioxide dissolves; burning them returns carbon mainly as carbon dioxide.
Match the process or store with its meaning.
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Condensation changes physical state; photosynthesis changes chemical substances; limestone and fossil fuels store carbon by different formation routes.
State what evidence can establish
- A modern volcano releasing carbon dioxide supports a possible volcanic origin. It does not show that Earth's first atmosphere was exactly 90% carbon dioxide. Independent geological evidence and planetary comparisons can strengthen an interpretation.
- Use the pattern “observation → supported explanation → limitation”. Treat the diagram as a qualitative sequence, not measured historical fractions or a time scale.
“Ancient ice directly samples the air when Earth first formed” is false. Ice records much later atmospheres. The earliest composition must be inferred from limited indirect evidence.
Today's proportions, cooling/condensation, ocean carbon removal, photosynthesis and burial explain different parts of the atmospheric change. Name the process and carbon/oxygen route, then state the evidence's limit.
Which two statements keep an early-atmosphere explanation defensible?
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State what was observed and what it supports, then identify the missing direct evidence. Exact historical percentages and universal planetary analogies overstate the evidence.